People are
always talking about how fast or slow the bass is of
loudspeakers, amps, preamps, even source components. You
especially hear how "slow" dynamic woofers are
compared to the high-speed of electrostatic panels,
especially when the two drivers are paired in a hybrid
loudspeaker. Comments often heard go something like this:
"Well, sure there is a speed difference between the
panel and the woofer. Nothing is as fast as a good
electrostatic panel. Expecting the woofer to keep up just
isnít realistic." Well, this month I will attempt
to explain just how wrong these snapshots of reality are.

The first thing we must
know is that bass itself is not particularly fast.
Virtually any woofer, even those with heavy cones
can easily reproduce bass frequencies with every
scintilla of speed present in that bass. So donít buy a
bunch of baloney about low-mass woofer cones leading to
"high-speed bass" -- it just isnít going to
happen. If the woofer can reproduce 40Hz with low
distortion, how fast the woofer starts is almost
irrelevant (within reason of course). It only needs to
accelerate fast enough to match the rise time of 40Hz at
the fastest point along a 40Hz sine wave. If the woofer
can do that, it is going as fast as it needs to in order
to be as fast as fast can be -- at 40Hz. The woofer cone
does not need to be able to accelerate at 20kHz
velocities in order to produce instantaneous 40Hz energy
and if you could build a woofer that
"fast," 40Hz would sound exactly the
same through a "slow" woofer.

Does this mean
that there is not such thing as fast bass and slow bass?
Absolutely not. It exists, just not for the reasons and
explanations you have been hearing for years, and
certainly not for the attributions youíve read in the
high-end press. There are reasons to use lighter,
lower-mass woofer cones. They just happen to be different
reasons than the ones youíve read in print. Smaller
woofers donít make faster bass, but they do reproduce
higher frequencies than larger woofers can reproduce, and
this is all important when it comes to speaker design.
You want the midrange driver and the woofer to integrate
with sublime symmetry, with perfection and with nary a
single problematic interaction throughout their overlap
zone. This is why you want smaller, lighter,
"faster" woofer cones -- not because they lead
to faster bass. That overlap zone is so amazingly
critical to your perception of bass speed that there is
little or no tolerance for error. The null tolerance for
integration error extends to phase, amplitude, frequency,
and time. Introduce even slight variations between any
part of the woofer and midrange (or panel) overlap zone
and you get audible effects in the bass or midbass. This
is where all of your perception of bass speed comes from.

In fact, bass
speed is virtually 100% a function of how ideally the
midrange and woofer are integrated. Bass linearity is
greatly involved also; you may see a flat
frequency-response curve, but the speaker can still sound
like it has lumpy bass response because of
less-than-ideal phase (or other) relationships between
the midrange driver and woofer. Phase can often change
with frequency. The woofer and midrange drivers can
actually veer off in different directions, phase-wise.
This is especially possible when you mix driver types
like panels and dynamic drivers. But large dynamic
drivers (woofers) operating at the top of their range and
medium-sized dynamic drivers operating at the bottom of
their range can often diverge significantly in their
phase response. When phase (or other) errors happen, you
get comb-filtering effects. This comb filtering results
in the complex response of the loudspeaker (to music)
being quite different than the response of the speaker
when the input is something simple like the sine-wave
sweep used to measure "frequency response."

To avoid
comb-filtering effects that cause "beating"
(reinforcement) and "cancellation" effects in
the sound (both are usually partial effects), it is
imperative for the phase, time domain, amplitude and
frequency performance of the woofer and midrange driver
to be "aligned." Get the midrange or woofer a
little ahead of or behind the other driver, and comb
filtering starts. You can do things to minimize it, but
you canít stop it with certain combinations of driver
and crossover. It is fearfully hard to integrate a
dynamic woofer with an electrostatic panel because the
two drivers are so different from one another. Your
absolute best shot is using an active crossover with
infinitely variable phase/frequency, polarity, time
domain and amplitude adjustments. Play with it long
enough and you could dial in the response of the dynamic
woofer and electrostatic panel to achieve perfection in
their integration. Achieving the same thing using a
passive crossover is incredibly difficult. Some designers
are getting better as they learn from years of trying,
but it is still one of the hardest things to do in audio
that I can imagine. Just getting a dynamic midrange and
dynamic woofer to integrate perfectly is enough of a
challenge. You can hear even small errors show up as
speed problems in the bass or midbass. These are the
kinds of "character" that will remain with the
speaker no matter where it is used.

How come one
speaker has so much more bass detail than another? This
too is strictly driver integration and NOT the quality of
the woofer itself, as you may have heard. The fact is,
bass detail comes from the midrange driver. But your
ear/brain is so completely fooled by this complex
interaction of midrange and bass sound that you believe
that it is strictly a bass-related thing. It isnít, and
you can prove it by listening to something very boring
but also very instructive. Listen to a subwoofer all by
itself for a while. You wonít hear anything vaguely
resembling speed coming from that slow, soggy-sounding,
plodding subwoofer. It has no detail and no speed
whatsoever when heard all by itself. Integrate it
carefully with a nice set of main speakers, however, and
suddenly the subwoofer has scads of detail, and if
the integration is off a little, the bass will sound fast
or slow too. All of that sense of speed and detail is
coming from the main speakers, but from the midrange, not
the woofer. That is why the integration of the woofer and
the midrange drivers is so critical to getting a
good-sounding speaker.

Another thing to
bear in mind: live bass does not sound fast or slow; it
just sounds like bass associated with whatever instrument
or other source is creating it. The concept of
"fast" or "slow" bass is a
loudspeaker and audio-system-related thing. Oh, I suppose
you could devise a live demonstration to show how the
midrange of a string bass can affect the perceived
quality of the bottom end of its range (and for all I
know, the best musicians may use this to further extend
their emotional reach in their playing). But in
day-to-day listening situations when you hear live music,
I doubt youíve ever thought about the "fast"
or "slow" bass that you were hearing. No,
thatís something that happens at home in the
reproduction chain, and itís an artifact of integration
errors. Remove the integration errors and the bass loses
all sense of being fast or slow, just like live bass.

For you this
means something profound. If you hear a system (hopefully
not yours) that sounds "fast" or
"slow" in the bass, enough that you have
noticed anyway, that system has a problem. It might be
fixable if the bass is coming from a subwoofer with lots
of adjustments. But most of the time, it will take some
minor or major change to remove the fast or slow
character. A different footer can affect apparent bass
speed because it changes the midrange of the
electronic component or loudspeaker it is used under, not
because it couples (or isolates) to the floor or shelf
better. The different foot simply changes the character
of the midrange a little bit, and because the midrange
and bass quality are so tightly intertwined, the quality
of the bass changes too, even though nothing specifically
changed in the bass itself.

So there you
have it, the symbiotic existence of bass and midrange,
which are more tightly interwoven and interdependent that
you may have thought.